(1) Field of the Invention
The invention generally relates to RF antennas and more specifically to bicone and dipole antennas.
(2) Description of the Prior Art
Resonant Antennas
Antennas that have two or more relatively isolated paths from their feed points to their ends, on two or more dimensions of a structure, or have two or more relatively isolated or independent elements, can have a resonance associated with each path or element. For example, the elements of one of the bifilar helixes of the quadrifilar helix described in U.S. Pat. No. 5,138,331 (herein incorporated by reference in its entirety for background information only), are an example of two or more relatively isolated elements.
If the resonances are adjacent to each other in frequency, they can combine to form a resonance/anti-resonance impedance loop with a bandwidth formed of the overlapping bands of both resonances. The overlapping bands can be formed in one of several ways. In one way, the feed points of two resonant elements can be feed in parallel, which results in an input impedance that is the parallel combination of the impedances of each element, where ideally the elements are fully isolated from each other. A resonance/antiresonance loop can form between the two resonant frequencies.
Infinite isolation occurs if the elements are separated by infinite distance, or if each element occupies its own dimension independent and located 90 degrees from other occupied dimensions. When elements are closer together, coupling between them occurs and bandwidth loss from the increase in the magnitude of the reflection coefficient (about the antenna characteristic impedance Z0) is possible. Much tighter spacing and coupling can also change reflection coefficient phase and impedance locus shape.
As an example, the elements of one of the bifilar helixes of the antenna of U.S. Pat. No. 5,138,331 were very tightly spaced at approximately 0.02 wavelength separation, because of space constraints. Also the configuration did not allow a second element to be rotated 90 degrees away into another dimension from a first element. Thus, the elements were tightly coupled to each other. This initially introduced a large Voltage Standing Wave Ratio (VSWR) spike in the band overlap area between any two resonance frequencies of any two adjacent elements fed in parallel, caused by a non-radiating transmission line formed between the elements. In the overlap area, the shorter element with a resonance frequency above the area has a capacitive impedance of negative phase approaching −90 degrees. The longer element with a resonance frequency below the area has an inductive impedance of positive phase approaching 90 degrees. The resultant phase difference approaches the 180 degrees of a transmission line. To rid the transmission line and spike, a feed phase difference of 180 degrees was introduced between the elements, to bring the approaching 180 degrees phase difference to approaching 0 degrees. In the areas of the resonances and their overlapping, the resultant impedance still had some loss of bandwidth from an increased reflection coefficient magnitude (about the antenna characteristic impedance Z0), when compared to the calculated parallel combination of the measured impedances of each element by themselves.
Another way overlapping bands can be formed is as follows. One resonant element can be fed with coupling to other parasitic elements resonant at adjacent frequencies. The impedance locus becomes that of the fed element, with the parasitic elements inserting resonance/antiresonance loops in the locus near the parasites' resonance frequencies, for an increase of bandwidth. This is further explained in the background in U.S. Pat. No. 6,118,406.
Another way overlapping bands can be formed is as follows. A parallel combined impedance resonance/antiresonance loop can result from the resonances of two or more paths on two or more independent 90 degree (referenced to a feed point) separated dimensions of an antenna of simple shape. This type of impedance is more complex, such as with the location of the resonance frequencies, because the parts of the antenna in each dimension are not fully isolated from each other. From the feed point, currents are not necessarily restricted from flowing only along the direction of the dimensions, but can flow also along paths between the dimensions. For example, for a flat rectangle dipole, currents can flow not only along the direction of the independent dimensions of length and width, but along a diagonal between these dimensions, from the feed point to a corner on the open end of the rectangle.
Ninety degrees of separation allows maximum isolation. However, antennas with geometries of maximum separations of less the 90 degrees can still have a resonance for the length at each extreme of the separation, but there will be less isolation and independence between each resonance, and less bandwidth. For example, the flared edges of a flat bicone with a flare angle of less than 90 degrees can be made with asymmetrical lengths. In the extreme case of 0 degrees of separation, there is only one dimension of one length, and thus only one possible resonance.
In general, a certain amount of electrical distance must exist between two paths for each path to be able to be a distinct resonant path. For paths along independent dimensions, this is afforded simply by the physical 90 degree separation between two paths. However, enough electrical separation between paths physically separated by less than 90 degrees also allows distinct resonant paths. This results in the fact that in general for a continuous structure, more resonant paths can exist on the structure as frequency is increased.
As frequency is increased, an initially resonant basic antenna tends to lose its resonant behavior as it becomes more broadband from the following factors:                a. resonant paths become wider and thus more broadband;        b. more paths become available from increased available electrical separation; There can be more different resonant lengths leading to more broadband behavior;        c. the antenna can start to become a traveling wave antenna from increased path lengths and from the condition where most of the energy applied at the feed point has radiated by the time it reaches the end of the antenna.        
Reflection coefficient and VSWR FIGS. 5 and 6 shows in more detail how ideally an overlapping band can be formed, using an example of a case of parallel combination of two ideal resonant bands of narrow bandwidth. Traces 301, 401 shows an ideal single ¼ wavelength resonance at 305, 405 plotted versus electrical length over the range of 0 to ½ wavelengths, and is representative of a simple open ¼ wavelength antenna element. It was generated by taking a 0.9 magnitude reflection coefficient circle and changing its normalized characteristic impedance Z0 so that the point where it crosses the real axis of the Smith Chart at its resonant point would lie at the center of the Smith Chart. The rest of the traces show the resultant impedance when the impedances of two such resonance traces are combined in parallel, with increasing degrees of difference of their resonance frequencies and band centers, in terms of differences in their electrical length. The electrical length shown is the average of lengths of both traces. Traces 302, 402 show the case when both resonant frequencies coincide, with a difference in electrical length of 0. Although the VSWR traces show an apparent increase in bandwidth, there is no difference in bandwidth. The apparent increase is due to a halving of the impedance with a resultant detuned VSWR, both due to the parallel method of combining the impedances. With a parasitic combination, this would not occur. Traces 303, 403 show a slight 0.0125 electrical length difference in resonances, with resonant points 306, 406 and 307, 407. Some increase in bandwidth is seen and a small, tight dip 311 in the shape of a V is seen in the impedance locus. Traces 304, 404 show a case of further separation of 0.025 at resonant points 308, 408 and 309, 409. More increase in bandwidth is seen, but with a rise in VSWR starting to occur at the center of the band. The tip of the V has started to form into a less tight resonance/antiresonance loop 312, with a spreading out of the locus. The apparent center of the two bands of the two resonances start to appear at 410 and 411, but the actual centers are at 408 and 409 (308 and 309 on the Smith Chart). The difference is due to the impedance combination mechanism and selection of the Smith Chart characteristic impedance. Further separation of the resonance frequencies would push the bands further apart, with a large VSWR occurring between the bands, and an increase in the size of the resonance/antiresonance loop. From another viewpoint, as the bandwidths of two adjacent resonances increases, there is more overlapping of bands, with the resonance/antiresonance loop of the overlapping area decreasing in size, from an example loop 312 of trace 304 to the indented V 311 of trace 303 to disappearing almost to a single spot on a trace similar to point 305 on trace 302. A large frequency range of the impedance locus would appear almost at a single point on the Smith Chart. Overall, larger resonant bands would allow larger combined bands.
For larger bands to be matched, more than two resonances such as for example from more than two resonant elements are used, with all element resonance frequencies staggered side-by-side across the band and all elements fed in parallel. For any given frequency in the band, only one element is radiating at resonance at low impedance or the areas near and between the adjacent resonances of two radiating elements are overlapping at low impedance. The resultant low impedance is only from these elements. All other elements are at high impedance with little effect on the resultant impedance, and with little radiation. Those resonant above the given frequency are shorter, with a high capacitive impedance; those resonant below the given frequency are longer with a high inductive impedance. For very large bands, all the elements will eventually radiate at their next higher order resonances, and thus the staggering of frequencies can continue with the frequencies of these resonances, subject to the patterns of these resonances and their resistive impedances being acceptable.
Ideally the resonance/antiresonance loop of any type of combination of adjacent resonances should be centered on the real axis of the Smith Chart. However, other variables of the antenna, such as the condition of dimensions and their associated resonances not being fully isolated from each other, or other components of capacitance, inductance, and delay reflective of the antenna geometry, can move and rotate the resonance impedances and resultant loop in the capacitive or inductive direction. For dimensional resonances, their frequency locations and combining can become complex, since continuous antenna elements have no distinct boundaries between the dimensions in which different sized pieces of an element can independently resonate. Also antenna shape largely determines locations of resonance paths.
The combination of two resonances of two largely different bandwidths can also cause the loop to be off of the real axis, where the narrow band resonance appears as a small loop almost anywhere in the impedance locus of the large band resonance. For example, a narrow band resonant length of a narrow feed cable feeding a broadband resonant fat dipole or broader band bicone antenna on its axis can insert a small resonance/antiresonance loop into the impedance locus of the antenna when the antenna is electrically small and of high impedance. For another example, slight irregularities such as asymmetries or cracks in an antenna can also create slightly different length narrow resonant paths that can add small loops anywhere in an antenna impedance.
Ideally the resonance/antiresonance loop should center about the feed Z0, typically 50 ohms. For simple antennas composed of two elements that have a width and length, this can be done to varying extents by controlling the antenna characteristic impedance Z0 where Z0 is given by
      Z    0    =                    L        C              .  where C is capacitance per antenna element unit length and L is inductance per antenna element unit length. Wider or fatter antenna elements will increase capacitance between elements and reduce inductance along their lengths, and thus reduce Z0.
Examples of two dimensional antennas whose dimensions can be at least roughly of resonant length are flat dipoles, the end fire slot, and the rectangular patch fed at the center of one of its edges. Even the three dimensional bicone fed dipole, discussed in detail later, can have two resonant dimensions, e.g. its diameter and its length from its feed point to its end, although usually the bicone aperture is open enough so that the impedance of the bicone part or diameter of the antenna is very broadband and almost nonresonant.
Inductively Shorted Bicone
A shorted bicone antenna 100 as illustrated in FIG. 3 includes a top cone 102, a bottom cone 104, a feed point 105 at a feed angle 106 between top cone 102 and bottom cone 104, a feed cable 107 to connect to feed point 105 and narrow inductive shorts 108 placed vertically across and connected to the outer edges 110, 112 of the cones of the antenna 100, allowing cables to pass the antenna vertically via the shorts. This allows other antennas to be mounted and fed above the shorted bicone. The effects of the shorts on the normal bicone are described below and in more detail in U.S. Pat. No. 6,268,834, herein incorporated by reference in its entirety for background information only.
The shorts somewhat increase the cut-in frequency of the bicone. The cut-in frequency is defined where and above which the voltage standing wave ratio (VSWR) about the antenna characteristic impedance Z0 becomes low and flat for infinite bandwidth. To minimize this rise, the shorts are made as narrow as practically possible and their number is minimized.
At lower frequencies, from roughly 1500 MHz to 0 Hz, where the radial path 118, 122 for example from the feed point to center 124 for example of the shorts becomes small, the shorts wrap the Smith Chart impedance locus of the open antenna progressively an extra half turn counterclockwise to a short at 0 Hz, when the open antenna is fully shorted. At 0 Hz the locus is wrapped an entire half turn from an open to a short. At somewhat higher frequencies from 1000 to 1700 MHz, where the path is no longer small, an increase of the bandwidth of the antenna occurs, from a significant tightening of the impedance locus in the shape of a shallow V. The tip of the V has a tiny loop at around 1100 MHz to 1300 MHz. This can be seen in the impedances of the antenna before and after addition of the shorts as shown in FIGS. 4A and 4B in U.S. Pat. No. 6,268,834. At higher frequencies above 2000 MHz, there is little difference in bandwidth with the addition of the shorts.
The V shaped part of the locus appears to be that of the V shaped part 311 of locus 303 of overlapping resonances of FIG. 5, except the resonances are separated slightly more which allows the resonance/antiresonance loop to form. The resonances can be from two different antenna parts or components.
The shorts force the antenna to have two parts: a shorted part and an open part. The shorted part consists of four shorted sections of roughly 45 degrees of circumference about the antenna axis 101 per section for half of the total circumference, with a short centered in each section. The open part consists four open sections between the shorted sections, of roughly 45 degrees of circumference about the antenna axis 101 per section for the other half of the circumference. The boundaries between the sections are not sharp but instead there is a smooth transition when moving circumferentially from one type of section to another. A section exists on both cones of the bicone, at the same circumferential positions on both cones. Half of a section on a given cone surface is pie shaped, extending from the feed point to the circumference of the antenna. An example of half of a shorted section is shown as the area 125 between radial lines 126, 127 and circumferential line 128.
The lengths of the open and shorted parts can be resonant. The length of the open part is the 2.74″ of example radial path 114 from the feed point 105 to an open edge point 116 of the bicone between two shorts. The length of the shorted part is the 2.74″ of the example radial path 118 from the feed point 105 to an end of a short at an open edge of the bicone at 120 and the length 1.37″ of example path 122 from point 120 to the center 124 of the attached example short, for a total length of 4.11″. If the resonant frequencies of the open and shorted lengths are adjacent, their bands can overlap and form the observed resultant V shaped part of the locus.
A given open end of the bicone and an adjacent shorted end are separated by 45 degrees of azimuth about the antenna axis, and thus there is at least some isolation between the two paths. With antennas with fewer shorts and wider sections, there is more isolation between the shorted and open parts, with more independence of the resonances.
Since a given part is half as wide in the circumferential direction as the original antenna's 360 degrees of circumference, its bandwidth can be expected to be roughly one half of the original antenna and be more resonant. The shorted part would have even less bandwidth because of the narrow band of its narrow shorts. The more resonant, narrower band impedances will change quicker with frequency than the very broadband resonances of the original bicone. Since the exact reduction in bandwidth of the parts is unknown, and since the parts are not fully isolated from each other, their exact resonance frequencies are unknown. However, at least the shorted resonance should be close to that of its path length, since the short itself is narrow and narrowband.
The following table shows what the resonant frequencies of the part lengths would be if they are narrow band, and any resonance/antiresonance loops between these frequencies observed in the impedance of the antenna in FIG. 4B. In parenthesis are the resonances of the original bicone. The V overlap area between the first two resonances is the most noticeable, because just above cut in, where the antenna becomes well matched and broadband, the antenna is most narrowband where narrowband loops can form. At higher frequencies, it is difficult to see any band overlapping since the antenna is more broadband and resonances are broadened significantly. Thus overlapping would produce at most only clustering of frequencies in the overlap area of the impedance locus, and not any indentations or loops. The first two resonance frequencies of 1078 and 1437 MHz of respectively the open and shorted sections are approximately in the same positions relative to the V as are resonance frequencies 306, 307 of theoretical trace 303 of FIG. 5. If instead the open section resonance frequency is lower and closer to the original open bicone resonance frequency of 695 MHz, then the shorted resonance has combined with a section of the open section impedance that is somewhat higher in frequency than its resonance frequency.
TABLE 1Resonantfrequency ofPath typepath or centerand length (wavelengths)Resonance/frequency ofOpen biconeShorted biconeantiresonanceloop (MHz)path 114path 118, 122loop1078 (695).251200Vindentation1437 .528741  3233 (2900).75
Overall, a tightening of the impedance locus occurred which appears to be from a band overlapping mechanism on the antenna, provided by at least one resonance due to a shorted path occurring near a resonance of the open parts of the antenna.
The shorts physically provide a balun path across the radiation aperture of the antenna, so other cables to antennas mounted above the shorted bicone can pass the bicone without affecting its performance. At higher frequencies where the distance between adjacent shorts becomes an appreciable part of a wavelength, nulls start to form in the azimuth patterns about the shorts, and increase in depth with increasing frequency.
Bicone Fed Dipole
FIG. 4 illustrates a bicone fed dipole 200 with a bicone feed section 203 that includes a top cone 202, a bottom cone 204, a feed point 205 at a feed angle 206 between top cone 202 and bottom cone 204, and with dipole sections 208, 209 of diameter 218 and height 220. The bottom edge of dipole section 208 and the top edge of cone 202 join and share common edge 210; the top edge of dipole section 209 and the bottom edge of cone 204 join and share common edge 212. Fat dipoles are commonly fed with bicones at their feed region. This allows the feed point impedance to more gradually taper to that of the dipole. If instead the feed region is simply two closely spaced parallel radial plates at the starting edges 210, 212 of the dipole cylinders 208, 209, formed of cones 202, 204 when feed angle 206 becomes 0 degrees, then the plates form a large shunt, non-radiating capacitance at the feed point 205 and across the resultant small gap between the flattened cone plates 202, 204 which decreases the impedance and the bandwidth of the antenna.
The bicone can help prevent the pattern splitting of a normal dipole. At lower frequencies, the bicone is small and radiates little and the antenna is thus a dipole composed of both the bicone and dipole sections, and having a dipole impedance radiating normal dipole patterns. Its whole length from the feed point 205 to the ends of the dipole sections 214, 216 radiates. This length is the whole length of radial dipole path 222 and 226 from feed point 205 to the end 228 of the dipole section, 208, and is also the length of the same path on the opposite cone section 204 and dipole section 209. At higher frequencies when a normal dipole is long enough (an approximately ¾ wavelengths path length) for elevation patterns to start to split on the horizon (the plane perpendicular to the antenna axis 201 at its feed point 205), if the bicone is large enough above its cut in frequency, it can start radiating a significant amount of the applied antenna power as its non-splitting bicone pattern. This radiation will help fill in the nulls of the dipole radiation. At even higher frequencies, by the time the wave introduced at the feed point reaches the end of the bicone at edges 210, 212, most of its power has been radiated before reaching the dipole sections 208, 209, and thus the antenna radiates patterns of a bicone, with an impedance mainly of the bicone by itself. Since a bicone with a small enough feed angle can maintain radiation on the horizon for a very large bandwidth, it can help stabilize the initial dipole patterns over a large bandwidth.
When the bicone is large, a significant part of the applied antenna power has radiated by the time the wave reaches the end 210, 212 of the bicone 202, 204 and the beginning 210, 212 of the dipole sections 208, 209. The impedance is mainly that of the bicone and any effects from the dipole sections 208, 209 is added to a bicone impedance. When the bicone is small, the whole antenna radiates with a dipole impedance, with its main effects being from the dipole sections 208, 209. These effects on the antenna impedance are described further below.
The addition of the dipole sections to the bicone gives the antenna two lengths that can possibly resonate or at least provide two bands. Radial path 222 from the feed point to the edge 210 of cone 202 at point 224 is the length of the bicone. This path plus its continuation as radial path 226 on the dipole section 208 to the end of the dipole section 214 at point 228 is a dipole length.
The details of the impedance behavior of a bicone fed dipole can be examined by looking at the impedance and VSWR (FIGS. 7, 8) of a sample bicone fed dipole of the following dimensions, as shown in FIG. 4:
TABLE 2ParameterValueBicone feed angle 20650 degreesAntenna diameter 2183.07″Dipole section height 2200″, curves 501, 521 of FIGS. 7, 82″, curves 502, 522 of FIGS. 7, 8Bicone radial path 222 length1.694″Dipole section path 226 length 2202″Dipole path 222 + 226 length3.694″Bicone radial path 222 length ¼1746 MHzwavelength frequency 508, 510Dipole path 222 + 226 length ¼799 MHzwavelength frequency 509Traces 501 and 521 show the impedance and VSWR of the bicone 202, 204 of the antenna by itself. Above a VSWR=3:1 cut in frequency 503, it shows a very broad band, low VSWR area 505, 525 typical of bicones, where the impedance locus wraps tightly around an antenna characteristic impedance 506. As the dipole sections are started to be added to the bicone, a tightening of the locus in area 505 above 1700 MHz starts to occur. This is due to two radiating lengths with two very close adjacent bands: a broadband bicone length 222 along the radial direction, and a somewhat longer narrower band dipole length composed of the bicone length 222 and the length 226 of the added dipole section. As the length of the dipole section is increased, a resonance/antiresonance loop starts to form in the impedance locus, reflecting the ¼ wavelength length resonance frequency 509 of the dipole, in a way similar to the formation of the loop between the two parallel resonances of trace 304 in FIG. 5, except the loop isn't necessarily near a resonance of the bicone impedance. This is more of a case of a narrow bandwidth resonance (of the dipole) inserting a loop into a broader band impedance (of a bicone). With further dipole section length, increased eventually to 2 inches, as the difference between the ¼ wavelength lengths of the bicone path 222 and dipole path 222 and 226 increases, this loop increases in size and moves down in frequency along the locus in direction 507, starting to separate itself and its band from the tight broadband bicone impedance in area 505. The tightness and bandwidth in area 505 reduces back to about what it was for the bicone by itself. Traces 502 and 522 show this situation, with the loop 511. The separation of the ¼ wavelength resonant length frequency 509 and its band of the dipole from the broadband area 505 of the bicone can allow a significant lowering of the cut in frequency 503 of the bicone to that of the bicone fed dipole (504) when dipole sections are added to the bicone, as is seen in FIG. 8. With even further lengthening of the dipole sections, the dipole band moves even lower in frequency and a VSWR spike starts to form between the dipole band and bicone band, similar to what is seen in trace 404 of FIG. 6. This eventually leads to formation of a two band antenna.
There continues to be a need for improved performance of antenna systems in all frequency ranges, specifically in the lower frequency ranges of bicone fed antenna systems.